1. Field of Invention
This invention relates to radar and more particularly to propagating a selected one of a plurality of patterns of radar beams.
2. Description of the Prior Art
A plurality of aircraft on a bombing mission, for example, typically include what is known as an electronic countermeasure screen aircraft. The screen aircraft usually carries a transponder that transmits a simulated radar return signal in response to receiving energy from a beam transmitted by a radar. The simulated return signal has a wave front that corresponds to a position where none of the aircraft are located. Therefore, the simulated return signal may prevent the radar from being used to determine positions of the aircraft.
When the radar beam is narrow, the transponder does not receive energy unless the radar beam is directed to the screen aircraft. Therefore, the narrow radar beam can be readily used to determine the positions of aircraft, other than the screen aircraft.
When the narrow radar beam has a high gain and is directed to the screen aircraft, a strong return signal is reflected from the screen aircraft. Because the reflected return signal is strong, it is easily distinguishable from the simulated return signal. Therefore, when the radar is used for determining the position of the aircraft, it is desirable for the radar beam to be narrow and have high gain.
When the radar is used for searching for the aircraft, it is desirable to propagate the radar beam through a large search region. Additionally, it is desirable that a reflected return signal have an amplitude that is independent of the range from the antenna of the radar of an aircraft that maintains a constant altitude. The amplitude of the reflected return signal is independent of the range when the radar beam forms the well known cosecant square pattern. The cosecant square beam pattern is formed when energy of the radar beam at a location in the search area is proportional to the square of the cosecant of the angle subtended by a line between the location and the antenna and a datum line that passes through the antenna. Accordingly, it is desirable that the radar have a capability for transmitting either the narrow high gain beam or the cosecant square beam pattern.